Method of coating low alloy steels

ABSTRACT

A method of fluxless hot dip metallic coating of low alloy steel strip and sheet containing aluminum, titanium, silicon, chromium, and/or mixtures thereof. A surface readily wettable by molten coating metal is obtained by heating the steel to a temperature of about 1100* to 1675* F in an atmosphere oxidizing to iron, then further treating under conditions which will reduce the iron oxide, whereby to form a surface layer comprising a substantially pure iron matrix containing a uniformly distributed fine dispersion of oxides of the alloying elements.

United States Patent [191 Flinchum et al.

METHOD OF COATING LOW ALLOY STEELS Inventors: Charles Flinchum, Trenton;F.

Curtiss Dunbar, Monroe; Jerry L. Arnold, Franklin, all of Ohio ArmcoSteel Corporation, Middletown, Ohio Filed: May 24, 1974 Appl. No.:473,142

Assignee:

US. Cl. 427/320; 427/319; 427/321; 427/431; 427/433; 427/444; l48/6.35

Int. Cl. C23C 1/00 Field of Search. 117/51, 114 R, 114 C, 114 A; 148/6,12 D; 29/1962 References Cited UNITED STATES PATENTS 4/1940 Sendzimir117/51 Dec. 9, 1975 3,295,199 1/1967 Schrader ..117/51x 3,320,085 5/1967Turner ..117/51 Primary ExaminerRalph S. Kendall Assistant ExaminerJohnD. Smith Attorney, Agent, or Firm-Melville, Strasser, Foster & Hoffman[57] ABSTRACT A method of fluxless hot dip metallic coating of low alloysteel strip and sheet containing aluminum, titanium, silicon, chromium,and/or mixtures thereof. A surface readily wettable by molten coatingmetal is obtained by heating the steel to a temperature of about llO0 to1675 F in an atmosphere oxidizing to iron, then further treating underconditions which will reduce the iron oxide, whereby to form a surfacelayer comprising a substantially pure iron matrix containing a uniformlydistributed fine dispersion of oxides of the alloying elements.

16 Claims, 4 Drawing Figures U.S. Patent Dec. 9, 1975 Sheet 1 of23,925,579 I UNREACTED MATRIX IvIe OXIDE SURFACE LAYER IRON OXIDE SURFACELAYER WITI-I Me OXIDE. DISPERSED OR IN OOLID OOLUTION A ORIGINAL SURFACESURFACE CONDITION AFTER PRETREATMENT C SURFACE CONDITION JUST PRIOR TOENTERING THE.

COATING BATH D GRAPHICAL REPRESENTATION OF M6 CONTENT" OF MATERIALIN'CONDITION O I Me CONTENT WEIGHT PERCENT ODISTANCE FROM suRFl ceCONVENTIONAL PROCESSING OF 5TEEL WITI-I Me (CRITICAL LEVEL Me. CONTENTWEIGHT PERCENT C o CONVENTIONAL PROCESSING OF STEEL WITH Me CRITICALLEVEL DISTANCE FROM SURFA E Me CONTENT WEIGHT PERCENT 0 MATERIALPROCESSED ACCORDING TO PRESENT INVENTION DISTANQE FROM SURFACE US.Patent Dec. 9, 1975 Sheet 2 of2 3,925,579

ExTE RN L 4 OXIDATION CRITICAL ALUMINUM CONTENT WEIGHT PERCENT hg kia DP30F.

O [0 2o 40 5 7O 8O (O0 METHOD OF COATING LOW ALLOY STEELS BACKGROUND OFTHE INVENTION 1. Field of the Invention This invention relates toimprovements in the process of hot dip metallic coating of low alloysteel strip and sheet material whereby to enhance the wettability of thesurfaces thereof by molten coating metals such as zinc, zinc alloys,aluminum, aluminum alloys and terne, and to insure good adherence of thecoating. Low alloy steels which may be treated by the process of thepresent invention include those containing up to about 3% aluminum, upto about 1% titanium, up to about 2% silicon, or up to about 5%chromium, and mixtures thereof, with the remainder of the compositiontypical of a carbon steel, as defined by Steel Products Manual, CarbonSheet Steel page 7 (May 1970) published by American Iron and SteelInstitute.

2. Description of the Prior Art In the hot dip metallic coating of steelstrip and sheet material without a flux, it is necessary to subject thesheet and strip to a preliminary treatment which provides a cleansurface free of iron oxide scale which is readily wettable by the moltencoating metal and to which the coating metal will adhere aftersolidification thereof. Two types of anneal-in-line preliminarytreatments are commonly used in this country, one being the so-calledSendzimir process and the other so-called Selas process. Detaileddescriptions of these two types of processes may be found in US Pat. No.2,110,893, issued Mar. 15, 1938 to T. Sendzimir, and U.S. Pat. No.3,320,085, issued May 16, 1967 to C. A. Turner, Jr., respectively.

Briefly, the conventional Sendzimir process for preparation of carbonsteel strip and sheet material for hot dip metallic coating involvespassing the material through an oxidizing furnace heated, withoutatmosphere control, to a temperature of about 1600F, by combustion,electric resistance, electric induction, or other conventional means,the residence time being sufficient to cause the material to reach atemperature of about 700 to 900 F, withdrawing the material from thefurnace into air, forming a controlled surface oxide layer varying inappearance from light yellow to blue, introducing the material into areducing furnace containing a hydrogen and nitrogen atmosphere, theresidence time being sufficient to bring the material to a temperatureof about 1350 to 1700 F. The controlled oxide layer is completelyreduced, and the material is then cooled to approximately thetemperature of the molten metal coating bath and led beneath the surfaceof the bath while surrounded by the hydrogen-nitrogen protectiveatmosphere.

In the conventional Selas method of treating carbon steel strip andsheet material, the steps comprise passing the material through afurnace heated to a temperature of at least about 2200F by directcombustion of fuel and air therein, the furnace containing an atmosphereof gaseous products of combustion having no free oxygen and at leastabout 3% excess combustibles, the residence time of the material beingsufficient to cause it to reach a temperature of about 800 to 1300 F,while maintaining bright steel surfaces completely free from oxidation,withdrawing the material from the furnace while still surrounded bygaseousproducts of combustion, introducing the material directly into are ducing section having a hydrogen and nitrogen atmosphere, in whichthe material may be further heated from 800 to 1700 F and/or cooled toapproximately molten coating metal bath temperature, and then leadingthe material beneath the surface of the bath while surrounded by thehydrogen-nitrogen protective atmosphere.

It has been found that the above-conventional processes, whilesatisfactory for treatment of carbon steel strip and sheet material, maynot satisfactorily prepare the surfaces of low alloy strip and sheetmaterial for hot dip metallic coating. More specifically, it has beenfound that low alloy steels containing aluminum, titanium, silicon,chromium and/or mixtures thereof in appreciable amounts are not wettableby molten coating metals such as aluminum and zinc when treated underthe above-described conditions. The final product is thus completelyunacceptable since large areas have no coating whatever or a coatingwhich does not adhere to the base metal.

Moreover, in carbon steel containing relatively small amounts ofalloying elements, e.g., about 0.05% acidsoluble aluminum, it has beenfound that poor adherence of the solidified coating matal to the basemetal occasionally occurs even though the material appears to have beenwetted by the molten coating metal. In other words, although themetallic coating is continuous, adherence is poor in some areas thereof,thus resulting in high rejection rates.

It is thus apparent that a definite need exists for a reliable processof fluxless hot dip metallic coating of low alloy steels which avoidsthe problems described above.

SUMMARY It is a principal object of the present invention to provide amethod for the hot dip metallic coating of low alloy strip and sheetmaterial which enhances the wettability of the surface of the materialby molten coating metal and which insures good adherence of the coatingmetal to the base material after solidification thereof.

In low alloy steels of the type defined above, the alloying elementaluminum (in uncombined form) is most easily oxidized, followed in orderby titanium, silicon, chromium, and iron. Conversely, iron oxide is themost easily reduced of these elements followed in order by the oxides ofchromium, silicon, titanium and aluminum. While not wishing to be boundby theory, it is applicants belief that conditions can exist in theconventional processing which would first result in the formation of anexternal skin of aluminum oxide, a refractory compound, which is notwettable either by molten zinc or by molten aluminum. If other elementssuch as titanium, silicon and chromium are present instead of aluminum,these may also diffuse or migrate to the surface and be oxidized to forma stable oxide layer which may not be wetted by the molten coatingmetal. Since aluminum oxide is extremely difficult to reduce, anysubsequent treatment under conventional carbon steel reducing conditionsis ineffective in producing a reduced surface layer which is wettable bythe molten coating metal.

The present invention constitutes a discovery that subjecting thesurface of a low alloy steel containing alloying elements more readilyoxidizable than iron to strongly oxidizing conditions in thepretreatment processing results in formation of a surface layer of ironoxide containing a dispersion of oxides of the alloying elements eitherin the form of relatively small, uniformly dispersed precipitates, or insolid solution. This 3 is followed by subjecting the steel to aconventional reducing treatment in a hydrogen-containing atmospherewhich reduces the surface layer to a substantially pure iron matrixcontaining a uniformly distributed fine dispersion of oxides of thealloying elements.

As used herein, the term internal oxidation will be understood todesignate the formation of a dispersion of oxides of alloying elementsin an iron matrix adjacent the surface, when processed conventionally.The term external oxidation will be used to designate the formation ofan external skin or layer of stable oxides of alloying elements morereadily oxidizable than iron, when subjected to conventional processing.However, these terms will not be applied to the process of the presentinvention.

In current commercial in-line-anneal hot dip metallic coating lines, therequired high degree of oxidizing po tential may be achieved as follows:

When practicing the Sendzimir process, the temperature of the strip andsheet material upon exiting the oxidizing furnace is increased to arange of about 1 100 to about 1400 F (rather than the conventional 700to 900 F). In the Selas process, the temperature of the strip and sheetmaterial exiting the direct fired preheat furnace is increased to about1400 to 1675 F (rather than the conventional 800 to 1300 F). Moreover,the atmosphere in the direct fired preheat furnace is modified so as tocontain to 6% excess oxygen and no excess combustibles.

The present application discloses that an external skin of unreducibleoxide will form in the reducing sections of both the conventionalSelas-type and Sendzimir processes if a critical level of alloyingelements is exceeded. As hereinafter explained in detail, Auger analysisshowed that this external oxidation also takes place in the pretreatmentfurnace of the conventional Selas process. In the conventional Sendzimirpretreatment processing the maximum temperature reached (900F) isbelieved to be too low for significant diffusion of the oxidizingelement to occur. Many of the alloy steels mentioned in this inventionare very resistant to oxidation, and, in fact, when a steel containingapproximately 2% Al, I, 2% Cr, 1% Si, 0.5% Ti is subjected to theconventional Sendzimir pretreatment practice, the maximum recommendedtemperature of 900F is insufficient to produce a visible oxide film.

The process of the present invention is unsuitable for a carbon steelwhich does not contain substantial amounts of the more easily oxidizedalloying elements because the iron surface would be scaled to such anextent that a conventional reducing treatment would not convert all ofthe thickness of the scale surface, and poor coating adherence wouldresult. It would likewise follow that a treatment for alloy levelsconsiderably lower than the above-mentioned 2% Al, 1, 2% Cr, 1% Si, 0.5%Ti steel, but yet beyond the carbon steel level, would requirepreoxidation treatment conditions between the maximum tolerable forcarbon steel and that required for the above cited example of a lowalloy steel.

The above theory also explains the previously-discussed problem of pooradherence of coating metal to a carbon steel base metal containingrelatively small amounts of acid-soluble aluminum, e.g., as little as0.03% in some instances. Here again the diffusion of aluminum to thesurface accompanied by oxidation thereof, while not forming an aluminumoxide layer of sufficient thickness or continuity to prevent completewetting of the surface by the molten coating metal, neverthelesssometimes prevents good adherence of the coating metal aftersolidification by reason of the refractory nature of the aluminum oxideareas on the surface.

In its broadest aspect, thee method of the invention can be relied uponto enhance to wettability by a molten coating metal of, and to insureadherence of the coating metal (after solidification thereof) to, thesurface of a low alloy steel containing one or more alloying elementsmore readily oxidizable than iron. This is effected by first heating thesteel to a temperature of about 1 to about 1675 F in an atmosphereoxidizing to iron, and subjecting the steel to further treatment underconditions which reduce the iron oxide, whereby to reduce the surfacelayer to a substantially pure iron matrix containing a uniformdispersion of oxides of the alloying elements.

BRIEF DESCRIPTION OF THE DRAWINGS Reference is made to the accompanyingdrawing wherein:

FIGS. 1A, 1B and 1C are diagrammatic representations of surfaceconditions at indicated processing stages of an iron alloy containing anelement Me, which forms an oxide more stable than iron oxide, in anamount less than the critical content under conventional Selas-typepretreatment conditions;

FIG. 1D is a graphic representation of the surface condition of thealloy of FIG. 1C;

FIGS. 2A, 2B and 2C are diagrammatic representations of surfaceconditions at indicated processing stages of an iron alloy containing anelement Me, which forms an oxide more stable than iron oxide, in anamount greater than the critical content under conventional Selas-typepretreatment conditions;

FIG. 20 is a graphic representation of the surface condition of thealloy of FIG. 2C;

FIGS. 3A, 3B and 3C are diagrammatic representations of surfaceconditions at indicated stages of the process of the present inventionof an iron alloy containing an element Me, which forms an oxide morestable than iron oxide, in an amount greater than the critical contentas calculated for conventional Selas-type pretreatment;

FIG. 30 is a graphic representation of the surface condition of thealloy of FIG. 3C; and

FIG. 4 is a graphic representation of the relation between the criticalaluminum content ofa low alloy steel and the hydrogen content and dewpoint of the treatment atmosphere.

DESCRIPTION OF THE PREFERRED EMBODIMENTS As stated above, according toapplicants theory, in conventional pretreatment of a low alloy steelcontaining an element which forms an oxide more stable than iron oxide,a surface layer of this more stable oxide is formed which is not reducedin the reducing section nor in the molten coating bath. Hence, verylittle wetting of the surface of the low alloy steel occurs. In thefollowing discussion, it should be kept in mind that it is necessary toqualify the term oxidizing to indicate whether it means oxidizing toiron. On the other hand, when the term reducing is used, this will meanthat it is reducing to iron unless otherwise specified.

As indicated above, the manner in which aluminum oxide is formed in alow alloy steel is of great significance. With low aluminumconcentrations, e.g., less than about 0.05% acid-soluble aluminum, and arelatively high oxidizing potential (such as that obtained by heating toabout 1800F in a hydrogen atmosphere having a dew point of about 120F)internal oxidation of the aluminum has been observed. Under thesecircumstances a precipitate of aluminum oxide is dispersed uniformly ina relatively pure iron matrix, and the surface of the alloy remainspredominantly pure iron. However, as the concentration of aluminum isincreased, or as the oxidizing potential is decreased, the rate ofpenetration of the internal oxide is decreased. At some combination ofaluminum content and relatively low surface oxidizing potential, atransition from internal to external oxidation will occur. This externaloxidation results in the formation of the previously mentioned aluminumoxide layer or skin which acts as a barrier to prevent wetting by amolten coating metal. Calculations will be set forth hereinafter showingthe relation between aluminum contents and oxidizing potential whichcauses the formation of such an aluminum oxide layer, but which can beavoided successfully in accordance with the present invention.

As explained above, the essential feature of the present invention is toconduct the oxidizing treatment under conditions which are highlyoxidizing to iron. This results in the formation of a surface layer orscale on the low alloy steel strip and sheet which is primarily ironoxide (Fe O in which oxides of alloying elements such as aluminum,titanium, silicon and chromium are present either as finely dispersedprecipitates or in a solid solution with iron oxide. In either eventthese stable oxides of the alloying elements are present as a minorvolume fraction of the surface layer and are uniformly dispersedthroughout the layer. In other words, diffusion or migration of thealloying elements to the surface is avoided. When material having asurface layer in this form is passed through the reducing furnace, theiron oxide portion is readily reduced. The more stable oxides of thealloying elements are not reduced andremain uniformly dispersedin asubstantially pure iron matrix. In this condition the low alloy steelsurface is readily wettable by a molten coating metal such as zinc oraluminum.

It is unlikely that a layer of aluminum oxide could subsequently form onthe outer surface of the steel in the reducing section since this couldoccur only if aluminum diffuses from the unreacted matrix out throughthe freshly formed substantially pure iron layer to the surface.Reaction kinetics would dictate against such an occurrence.

After reduction of the iron oxide the hot dip coating process isconducted in conventional manner with the strip and sheet material beingled beneath the surface while surrounded by a protective atmosphere.Coating and finishing are effected by any conventional method.

A sample from a heat of a low alloy steel having a nominal compositionof about 0.05% carbon, 2% chromium, 2% aluminum, 1% silicon, 0.5%titanium, about 0.3% manganese and remainder substantially iron, wassubjected to a conventional Selas process of heating to about l200F withan atmosphere containing 3% excess combustibles, followed by treatmentin a reducing section at about 1600F for three minutes in an atmosphereof hydrogen and 75% nitrogen, having a dew point of -60F. Another sampleof the same heat was treated in accordance with the method of thepresent invention by heating to a temperature of 1500F in a direct firedfurnace having no combustibles and 2% excess 0 followed by the sametreatment in the reducing furnace as that set forth above.

These samples were subjected to surface analysis by an AugerSpectrometer made by Physical Electronics, Inc. An Auger spectrum wasobtained for each sample surface. Each sample was then sputter etchedwith an argon ion gun, and simultaneously the amounts of certainelements present were monitored using the multiplexing feature of thesystem. This gave an elemental concentration profile as a function ofdepth from the surface of each sample. After a certain period of sputteretching a second Auger spectrum was run for comparison with the initialsurface spectrum.

The most marked difference between the two Auger spectra of the initialsurfaces of each sample was that the surface of the conventionallytreated sample showed about 10 times more aluminum, less iron andslightly more oxygen present than did the surface of the sample treatedin accordance with the present invention. After sputter etching for 15minutes at a nominal 80 A/min rate, the conventionally treated sampleshowed significantly less aluminum and oxygen and niore iron than theinitial surface of that sample. After sputter etching the sample treatedin accordance with the method of the invention for 12 minutes at anominal 25 A/min rate, this sample showed little change in the aluminumcontent as compared to its initial surface, although iron increased andoxygen decreased substantially.

Reference is made to FIG. 2C, which represents diagrammatically, and toFIG. 2D, which represents graphically, the surface condition of theabove sample subjected to conventional Selas-type treatment, derivedfrom the data of the Auger spectra. It will be noted that a layer ofoxides of the alloying elements is formed on the surface of the-sample(i.e., external oxidation), whereas the alloy content drops sharply to alower value of short distance inwardly from the surface (FIG. 2D). Thisshows the diffusion or migration of alloying elements to the surface.Thereafter, as distance from the surface increases, the content of thealloying elements gradually increases, thus showing some tendency foralloying elements in the internal lattice of the steel to diffuse to thesurface. This is to be contrasted with FIGS. 1C and 1D showing thebehavior of a sample containing less than a critical content and thusexhibiting internal oxidiation.

Reference is made to FIG. 3B representing diagrammatically the surfacecondition 'of the above sample after heating in an atmosphere oxdizingto iron in accordance with the process of the invention. A surface layeris formed comprising iron oxide and oxides of the alloying elementsuniformly dispersed, or in solid solution, in the iron oxide layer. FIG.3C represents diagrammatically, and FIG. 3D represents graphically, thesurface condition after the reducing treatment, derived from the data ofthe Auger spectra. FIG. 3D shows that the concentration of alloyingelements at the surface is substantially less than in the correspondingstage of the conventional treatment shown in FIG. 2D.

The mathematics for internal oxidation have been established in thefollowing articles:

C. Wagner, Zeit. Elekrochem., 63, pp 772-790 (1959 R. A. Rapp,Corrosion, 21, pp 382-401 (1965) J. H. Swisher, Oxidation of MetalsandAlloys, pp

235-267, ASM (1971) In order to permit simplification in themathematics, a special case will be assumed in which aluminum is thealloying element and in which:

5 No) D NM" Du l where 1 NJ" oxygen mole fraction established at thesurface N original mole fraction soluble Al D diffusivity of Al Ddiffusivity of O The rate of internal oxidation is given by 70 00 where5 depth of penetration of internal oxide t time 112 X DUI/2 x Natl) V,-,molar volume of body-centered-cubic iron, VMo3/2 molar volume of Al O(i.e. one half of molar volume of M 0 Assuming a temperature of l600Fand an atmosphere of 25% hydrogen and 75% nitrogen with a dew point of-60F the oxygen partial pressure is calculated to be 1.08 X 10 24atmosphere. Using results published in an article by J.l-l. Swisher andE.G. Turkdogon in Trans. Met. Soc. AIME, 239, pp. 426-431 (1967) on thesolubility of oxygen in body-centered-cubic iron, a value of equilibriumoxygen solubility (N of 2.73 X 10'9 is obtained, Using puslished data?for D D V and V 0 a value of 0.05% aluminum is calculated whichrepresents the critical level for the above operating conditions. Morethan 0.05% aluminum would result in an external aluminum oxide layer orscale, while less than 0.05% aluminum would produce an internal oxide ofaluminum oxide precipitated uniformly in an iron matrix. Bester & Lange,Arch. Eisenhuttenwes" 43, 207-213 (I972) Vignes et al, Trans. 2nd Natl.Conference Electron Microprobe Analysis" Paper No. 20, Boston (I967)Kubaschewski and Hopkins, 'Oxidation of Metals and Alloys" 7, l lButterworths (1967) As will be apparent from the above equation, anincrease in the dew point of the gas (which would in crease N wouldresult in an increase in the critical aluminum content which could betolerated and still avoid formation of an external aluminum oxide scale.In other words, a higher oxidizing potential raises the criticalaluminum content.

Reference is made to FIG. 4 which is a graphic representation of therelation of hydrogen content and dew point to the critical aluminumcontent in body-cen- 0 tered-cubic iron at a temperature of l600F. Analuminum content in the area beneath each curve results in internaloxidation, while an aluminum content above each curve results inexternal oxidation with consequent formation of a difficulty reducibleoxide layer or scale. The curves of FIG. 4 are plotted from equation (1above. It is apparent that relatively slight increases in the hydrogencontent sharply reduce the critical aluminum content at the lowerhydrogen levels.

It is desired to emphasize at this point that the above equation and thegraph of FIG. 4 are not a definition of or limitation on the presentinvention. Rather, these make it possible to predict in a quantitativemanner when and why external oxidation may occur in conventionalfluxless hot dip metallic coating operations. The present inventionmakes it possible to avoid external oxidation when the critical aluminumcontent exceeds that which could be tolerated under conventional ornormal conditions. In other words, the equation and graph of FIG. 4 canbe used to ascertain whether a steel of any given composition may beprocessed in conventional manner or whether it must be processed inaccordance with the present invention in order to obtain goodwettability by the molten coating metal and good adherence of thecoating.

The above equation, while not exact, can also be utilized (withappropriate substitutions) to calculate the concentrations of otherelements such as titanium, silicon and chromium, which form oxides morestable than iron oxide. If more than one of such elements is present,the critical content of the element which forms the most stable oxide(aluminum) should first be calculated, followed in order by calculationsof the critical contents of titanium, silicon and chromium. If none ispresent in an amount near the critical content, external oxidationshould not occur under conventional processing conditions unless two ormore elements exhibit a synergistic or cumulative effect, with thefractions of critical contents adding up to a total greater than thecritical content of any one element.

A coil of strip of the above 2% Cr 2% Al 1% Si- 0.5% Ti steel treated inaccordance with the method of the invention was coated in a Selas-typecommercial aluminum coating line. The strip surface was readily wettedby the molten aluminum, and the solidified coating exhibited excellentadherence to the base metal strip.

For comparison, another coil of the same low alloy steel was subjectedto conventional pretreatment followed by coating in the same commercialaluminum coating line. This strip was not wettable by the moltenaluminum, and the final product was thus unacceptable. The.treatmentconditions for these coils are summarized in Table I.

TABLE 1 Low Alloy Strip .050 in thickness X 48 in Width nominal 2% Cr,2% Al, 1% Si, 0.5% Ti, 0.5% C, 0.3% Mn, balance Fe Present Invention 1stTrial 2nd Trial Pretreatment Conditions Conventional Practice (1)Combustion Ratios preheat section 2.6% excess 0 of Slow Cool Zone 2.8%excess 0 4% excess gas (combustible) 1200F 1750F 193 fpm 1S00F 3500 cfhA series of nine laboratory heats was prepared with pure iron as a baseand to each of which a different amount of aluminum or silicon wasadded. These samples were then rolled to strip thickness and coated withmolten aluminum in a Selas-type continuous coating line. Furnaceconditions were in accordance with conventional practice in that thedirect fired preheat furnace atmosphere contained 6% combustibles andthe temperature to which the strips were heated in the preheat furnacewas 1275F. Critical contents of aluminum and silicon were calculatedfrom equation (1) above for the furnace conditions.

Metallographic examination of the coated samples showed that in allinstances where the aluminum or silicon content was less than thetheoretical critical amount, as determined from equation (1), thematerials were completely wetted by the molten aluminum of 35 thecoating bath. In all cases where the aluminum or silicon content wasequal to or greater than the theoretical critical content, metallurgicalexamination showed a lack of wetting as evidenced by areas which did notcontain an iron-aluminum intermetallic alloy layer.

Additional samples from all nine heats were then coated on the sameSelas-type coating line under furnace conditions contemplated in theprocess of the present invention. The preheater was adjusted to provide3% excess 0 and no combustibles in the furnace atmosphere and sampleswere heated to slightly above 1500F, thereby creating conditionsstrongly oxidizing to iron. These furnace'conditions resulted incomplete wetting by molten aluminum of all heats, even those whichexhibited uncoated areas under conventional processing conditions. Theresults of these tests are summarized in Table II.

It is apparent from these tests that merely heating a steel toatemperature above that used in conventional processing is not effectiveif the atmosphere is not oxidizing to iron at the temperature involved.It is further evident from the Auger spectra reported above that theprocess is equally effective for aluminum-and/or-silicon-killed steelsand for steels containing greater amounts of alloying elements, e.g., upto about 3% aluminum, up to about 5% chromium, up to about 2% silicon,up to about 1% titanium, and mixtures thereof. Moreover, although theprocess of the invention has particular utility in aluminizing steelscontaining the specific alloying elements recited above, it is not solimited and is effective for fluxless hot-dip coating by anycommonly-used coating metal of a ferrous metal strip or sheet containingan alloying element or elements more readily oxidizable than iron.

Coating metals which may be used include, but are not limited to, thosedescribed in U.S. Pat. No. 2,784,122 issued Mar. 5, 1957 to N. Cox etal, at column 2, lines 9-33; and in US. Pat. 2,839,455, issued June 17,1958 to H. La Tour et al, at column 1, lines 68-72 and column 2, lines1-7. The disclosures of these patents are incorporated herein byreference.

TABLE II Aluminum-Coated Low Alloy Steels Metallographic Examination ForCoating-Base Metal Diffusion Layer CONDITION I Samples Al (0.15% A1Critica1)* CONDITION Il A-l 0.008 Good coating" Good coating A-2 0.036Good coating Good coating A-3 0.22 Uncoated areas*** Good coating Si(0.41% Si Critical)* 8-1 1.28 Uncoated areas Good coating 8-2 0.18 Goodcoating Good coating B-3 0.027 Good coating Good coating Si (0.41% SiCritical)* C-l 0.70 Uncoated areas Good coating C-2 0.12 Good coatingGood coating C-3 0.003 Good coating Good coating TREATMENT CONDITIONSPreheater Reducing Furnace Critical Contents* STRIP TEMP. MAXIMUM IAFTER EXCESS O H D.P. STRIP TEMP. Al it PREHEATER COMBUSTIBLES Cond.l1275F 6 0 100 +15F 1500a 0.15 0.41

TABLE II-continued Aluminum-Coated Low Alloy Steels MetallographicExamination For Coating-Base Metal Diffusion Layer Samples Al Cond. IIlOOF 0 3 As indicated above, in its broadest aspects, the method of theinvention comprises heating a low alloy steel containing alloyingelements more readily oxidizable than iron in an atmosphere oxidizing toiron under conditions which form on the steel a surface layer of ironoxide containing a dispersion of oxides of the alloying elements, thenfurther treating the steel under conditions reducing to iron oxide. Whenthe initial heating step is carried out in accordance with the Selastypeprocess, the steel is preferably heated to a temperature of about 1400Fto about 1600F in an atmosphere of gaseous products of combustioncontaining 0% to 6% excess 0 preferably about 2% excess 0 and nocombustibles. In the subsequent reducing section the steel is preferablybrought to a temperature of about l500 to about 1700 F in an atmospherecontaining hydrogen, preferably at least about 20% hydrogen. The steelis then cooled to appropriate bath entry temperature while stillprotected by the hydrogennitrogen atmosphere, the dew point of whichmust be consistent with carbon steel practice.

It will be understood that the strip bath entry temperature and maximumdew point of the hydrogen-nitrogen atmosphere in the furnace aredependent on the type of coating metal (i.e., the minimum striptemperature prior to bath entry). In general the strip is brought to atemperature ranging from slightly less than to slightly higher than thatof the coating metal bath. When coating with aluminum a dew point nothigher than about 50F should be observed. When galvanizing, a maximumdew point of about F should be observed because of the lower striptemperature. For aluminizing, typical strip bath entry temperatures areabout 1250F to 1350F, while for galvanizing, typical strip bath entrytemperatures are about 850 to 950 F.

In a new installation the advantages of rapid strip heating,adaptability to processing different types of steel, and furnacepressure control clearly favor the use of a Selas-type installation.However, as indicated above, the method of the invention is equallyapplicable to a Sendzimir-type process, and existing installations ofthis type can be readily adaptedfor operation in accordance with themethod of this invention. Basically, the only difference is to heat thesteel in the oxidizing furnace to a temperature of l 100F or greater,preferably to 1300F. The conditions in the reducing section remainunchanged.

The lower strip preheat oxidizing temperature range for theSendzimir-type process as compared to the Selas-type process isaccounted for by the differences in atmosphere composition to which thestrip is exposed. Thus, to produce a given thickness of surface oxide, alower temperature is required when strip is heated in the Sendzimiroxidizing furnace and exposed to air than with the Selas-type systemwhere the strip is exposed only to oxidizing products of combustionprior to direct entry into the reducing furnace.

CONDITION l (0.15% Al Critical)* CONDITION ll The embodiments of theinvention in which an exclusive property or privilege is claimed aredefined as follows:

l. A method of enhancing the wettability by a molten coating metal ofthe surface of a low alloy steel strip and sheet stock containingalloying elements more readily oxidizable than iron, chosen from thegroup consisting of aluminum, titanium, silicon, chromium, and mixturesthereof, said alloying elements being present in amounts greater thanthe critical contents thereof as calculated from the following equationwherein aluminum and aluminum oxide are used as illustrative of thealloying element:

Z! original mule fraction raluble A! D0 dllfiutrltu o! oxunrn I N oxygenmole fraction established at the surface V molar volume ofbody-centered-cubic iron :1 stoichiometric ratio of oxygen to aluminiumatoms in M 0; D diffusivity of Al V molar volume of Alon/2 comprisingthe steps of passing said stock continuously through a furnace in whichsaid stock is heated to a temperature of about llO0 to 1675F in anatmosphere oxidizing to iron whereby to form on said stock a surfacelayer of iron oxide containing oxides of said alloying elements,dispersed or in solid solution therein, and subjecting said stock tofurther heat treatment in a hydrogen-containing atmosphere having adewpoint which makes said atmosphere reducing to iron oxide within thetemperature range of 800 to l700F whereby to reduce said surface layerto a substantially pure iron matrix containing a uniform fine dispersionof said oxides of said alloying elements.

2. The method claimed in claim 1, wherein said furnace is heated bydirect combustion of fuel and air therein to produce an atmosphere ofgaseous products of combustion containing 0 to 6% excess oxygen and noexcess combustibles, and wherein said stock is withdrawn from saidfurnace while still surrounded by said atmosphere at a temperature ofabout 1400 to about 1675F.

3. The method claimed in claim 2, wherein said coating metal isaluminum, zinc, or alloys thereof, and wherein said steel, afterwithdrawal from said furnace, is brought to a temperature of about 1500Fto about l700F in a hydrogen-nitrogen atmosphere comprising at leastabout 20% hydrogen.

4. The method claimed in claim 3, wherein said coating metal is aluminumor alloys thereof, wherein said steel is cooled approximately to thetemperature of the molten coating metal bath and introduced into saidbath while still surrounded by said hydrogen-nitrogen atmosphere, saidatmosphere having a maximum dew- 13 point of about 50F.

5. The method claimed in claim 3, wherein said coating metal is zinc oralloys thereof, wherein said steel is cooled approximately to thetemperature of the molten coating metal bath while still surrounded bysaid hydrogen-nitrogen atmosphere, said atmosphere having a maximumdewpoint of about F.

6. The method claimed in claim 1, wherein said furnace is heated withoutatmosphere control, and wherein said stock is withdrawn from saidfurnace into air at a temperature of about 1 100 to about l400F.

7. The method claimed in claim 6, wherein said coating metal isaluminum, zinc, or alloys thereof, and

- wherein said steel, after contacting air, is brought to a temperatureof about 1500 to about 1700 F in a hydrogen-nitrogen atmospherecomprising at least about hydrogen.

8. The method claimed in claim 7, wherein said coating metal is aluminumor alloys thereof, wherein said steel is cooled approximately to thetemperature of the molten coating metal bath and introduced into saidbath while still surrounded by said hydrogen-nitrogen atmosphere, saidatmosphere having a maximum dewpoint of about 50F.

9. The method claimed in claim 7, wherein said coating metal is zinc oralloys thereof, wherein said steel is cooled approximately to thetemperature of the molten coating metal bath while still surrounded bysaid hydrogen-nitrogen atmosphere, said atmosphere having a maximumdewpoint of about 15F.

10. The method of claim 1, wherein said low alloy steel contains up toabout 3% aluminum, up to about 1% titanium, up to about 2% silicon, andup to about 5% chromium.

11. The method of claim 10, wherein said furnace is heated by directcombustion of fuel and air therein to produce an atmosphere of gaseousproducts of combustion containing 0% to 6% excess oxygen and no excesscombustibles, and wherein said stock is withdrawn from said furnacewhile still surrounded by said atmosphere at a temperature of about l400to about l675F.

12. The method claimed in claim 10, wherein said furnace is heatedwithout atmosphere control, and wherein said stock is withdrawn fromsaid furnace into air at a temperature of about l100 to about l400F.

13. The method claimed in claim 10, wherein said coating metal isaluminum, zinc, or alloy thereof.

14. In the method of fluxless hot dip metallic coating of low alloysteel strip and sheet stock containing at least one alloying element inuncombined form chosen from the group consisting of up to about 3%aluminum,

11 stoichiometric ratio of oxygen to aluminum atoms in A1 0 DA!dimulvflv 0! Al V molar volume of A10 3/2 wherein the surface of saidstock is prepared for coating by a continuous preliminary treatmentinvolving heating under conditions producing an oxide coating on saidsurface, followed by further heat treatment under conditions reducing toiron oxide, and wherein the stock is thereafter passed into a moltenmetal coating bath while surrounded by a protective atmosphere, theimprovement which comprises heating said stock in the first said heatingstep to a temperature of about l to about 1675 F in an atmosphereoxidizing to iron whereby to produce a surface layer of iron oxidecontaining a uniform dispersion or solid solution of oxides of saidalloying elements.

15. The method of claim 14, wherein said first heating step is conductedin a furnace heated by direct combustion of fuel and air therein and inan atmosphere of gaseous products of combustion containing 0 to 6%oxygen and no excess combustibles, and wherein said stock is withdrawnfrom said furnace while still surrounded by said atmosphere at atemperature of about l400 to about 1675 F- 16. The method claimed inclaim 14, wherein said first heating step is conducted in a furnacewithout atmosphere control, and wherein said stock is withdrawn fromsaid furnace into air at a temperature of about 1 100 to about l400 F.

1. A method of enhancing the wettability by a molten coating metal ofthe surface of a low alloy steel strip and sheet stock containingalloying elements more readily oxidizable than iron, chosen from thegroup consisting of aluminum, titanium, silicon, chromium, and mixturesthereof, said alloying elements being present in amounts greater thanthe critical contents thereof as calculated from the following equationwherein aluminum and aluminum oxide are used as illustrative of thealloying element:
 2. The method claimed in claim 1, wherein said furnaceis heated by direct combustion of fuel and air therein to produce anatmosphere of gaseous products of combustion containing 0 to 6% excessoxygen and no excess combustibles, and wherein said stock is withdrawnfrom said furnace while still surrounded by said atmosphere at atemperature of about 1400* to about 1675*F.
 3. The method claimed inclaim 2, wherein said coating metal is aluminum, zinc, or alloysthereof, and wherein said steel, after withdrawal from said furnace, isbrought to a temperature of about 1500*F to about 1700*F in ahydrogen-nitrogen atmosphere comprising at least about 20% hydrogen. 4.The method claimed in claim 3, wherein said coating metal is aluminum oralloys thereof, wherein said steel is cooled approximately to thetemperature of the molten coating metal bath and introduced into saidbath while still surrounded by said hydrogen-nitrogen atmosphere, saidatmosphere having a maximum dewpoint of about 50*F.
 5. The methodclaimed in claim 3, wherein said coating metal is zinc or alloysthereof, wherein said steel is cooled approximately to the temperatureof the molten coating metal bath while still surrounded by saidhydrogen-nitrogen atmosphere, said atmosphere having a maximum dewpointof about 15*F.
 6. The method claimed in claim 1, wherein said furnace isheated without atmosphere control, and wherein said stock is withdrawnfrom said furnace into air at a temperature of about 1100* to about1400*F.
 7. The method claimed in claim 6, wherein said coating metal isaluminum, zinc, or alloys thereof, and wherein said steel, aftercontacting air, is brought to a temperature of about 1500* to about1700* F in a hydrogen-nitrogen atmosphere comprising at least about 20%hydrogen.
 8. The method claimed in claim 7, wherein said coating metalis aluminum or alloys thereof, wherein said steel is cooledapproximately to the temperature of the molten coating metal bath andintroduced into said bath while still surrounded by saidhydrogen-nitrogen atmosphere, said atmosphere having a maximum dewpointof about 50*F.
 9. The method claimed in claim 7, wherein said coatingmetal is zinc or alloys thereof, wherein said steel is cooledapproximately to the temperature of the molten coating metal bath whilestill surrounded by said hydrogen-nitrogen atmosphere, said atmospherehaving a maximum dewpoint of about 15*F.
 10. The method of claim 1,wherein said low alloy steel contains up to about 3% aluminum, up toabout 1% titanium, up to about 2% silicon, and up to about 5% chromium.11. The method of claim 10, wherein said furnace is heated by directcombustion of fuel and air therein to produce an atmosphere of gaseousproducts of combustion containing 0% to 6% excess oxygen and no excesscombustibles, and wherein said stock is withdrawn from said furnacewhile still surrounded by said atmosphere at a temperature of about1400* to about 1675*F.
 12. The method claimed in claim 10, wherein saidfurnace is heated without atmosphere control, and wherein said stock iswithdrawn from said furnace into air at a temperature of about 1100* toabout 1400*F.
 13. The method claimed in claim 10, wherein said coatingmetal is aluminum, zinc, or alloy thereof.
 14. IN THE METHOD OF FLUXLESSHOT DIP METALLIC COATING OF LOW ALLOY STEEL STRIP AND SHEET STOCKCONTAINING AT LEAST ONE ALLOYIN ELEMENT IN UNCOMBINED FORM CHOSEN FROMTHE GROUP CONSISTING OF UP TO ABOUT 3% ALUMINUM, UP TO ABOUT 1%TITANIUM, UP TO ABOUT 2% SILICON, UP TO ABOUT 5% CHROMIUM, AND MIXTURESTHEREOF, SAID ALLOYING ELEMENT BEING PRESENT IN AN AMOUNT GREATER THANTHE CRITICAL CONTENT THEREOF AS CALCULATED FROM THE FOLLOWING EQUATIONWHEREIN ALUMINUM AND ALUMINUM OXIDE ARE USED AS ILLUSTRATIVE OF THEALLOYING ELEMENT: NAL = ((0.3XPIXDOXNO(3)XVFE)/(2XVDALXVALO3/2))**1/2WHERE NA1=ORIGINAL MOLE FRACTION SOLUBLE AL DO=DIFFUSIVITY OF OXYGENNO(3)=OXYGEN MOLE FRACTION ESTABLISHED AT THE SURFACE VFE=MOLAR VOLUMEOF BODY-CENTERED-CUBIC IRON V = STOICHIOMETRIC RATIO OFOXYGEN TOALUMINUM ATOMS IN AI2O3 1)AI=DIFFUSIVITY OF AI VA103 2=MOLAR VOLUME OFALO3 2 WHEREIN THE SURFACE OF SAID STOCK IS PREPARED FOR COATING BY ACONTINUOUS PRELIMINARY TREATMENT INVOLVING HEATING UNDER CONDITIONSPRODUCING AN OXIDE COATING ON SAID SURFACE, FOLLOWED BY FURTHER HEATTREATMENT UNDER CONDITIONS REDUCING TO IRON OXIDE, AND WHEREIN THE STOCKIS THEREAFTER PASSED INTO A MOLTEN METAL COATING BATH WHILE SURROUNDEDBY A PROTECTIVE ATMOSPHERE, THE IMPROVEMENT WHICH COMPRISES HEATING SAIDSTOCK IN THE FIRST HEATING STEP TO A TEMPERATURE OF ABOUT 1100* TO ABOUT1675*F IN AN ATMOSPHERE OXIDIZING TO IRON WHEREBY TO PRODUCE A SURFACELAYER OF IRON IOXIDIZING CONTAINING A UNIFORM DISPERSION OR SOLIDSOLUTION OF OXIDES OF SAID ALLOYING ELEMENTS.
 15. The method of claim14, wherein said first heating step is conducted in a furnace heated bydirect combustion of fuel and air therein and in an atmosphere ofgaseous products of combustion containing 0 to 6% oxygen and no excesscombustibles, and wherein said stock is withdrawn from said furnacewhile still surrounded by said atmosphere at a temperature of about1400* to about 1675* F.
 16. The method claimed in claim 14, wherein saidfirst heating step is conducted in a furnace without atmosphere control,and wherein said stock is withdrawn from said furnace into air at atemperature of about 1100* to about 1400* F.